The present invention relates to counting apparatus particularly useful for rate counting in connection with a positioning servo system.
Counters both of the hardware and software type have been used for years in connection with not only tallying events, but in control applications. Generally, high rate counting is achieved through so-called hardware counters; i.e., a combination of logic switching circuits used and interconnected in accordance with a predetermined counting algorithm. Where large count fields are desired, such counters have been connected to programmed data processors. The operation generally required an interrupt of the data processor to respond to the counter; that is, a carry, borrow or other type of overflow signal from the hardware counter would interrupt the execution of programs in the data processor. The data processor, in turn, would then read a buffer containing measurement data to be written on a recording medium such as magnetic tape or magnetic disk, or the contents of the counter could be read into the data processor.
In certain control applications programmable controllers taking the form of microprocessors or microcomputers have severe cost constraints. Therefore, the compute power of the microprocessor and its ability to rapidly execute a program is limited. When a data processor is interrupt driven, it must have capacity in excess of what many microprocessors presently offer. Therefore, it is highly undesirable in a low cost controller application of a programmed data processor, of the microcomputer type for example, to be entirely interrupt driven for controlling diverse apparatus. This avoidance of the interrupt driver is particularly advantageous in positioning systems having stop/lock conditions where the members being controlled are subject oscillations about a reference point. The counter tracking the positional movements may go from all ones to all zeros and back again, having a series of borrows and carries which would dominate the operation of the programmed data processor. Such domination could prevent the processor from handling other time dependent control functions. Accordingly, rather than adding a larger counter which, while providing precision, can affect the servo characteristics in an adverse manner, other more viable low cost solutions are desired.